A variety of mammalian cells initiate DNA repair synthesis in response to damage by chemical mutagens and carcinogens. Usually this process is studied by following the incorporation of radioactive precursors into non-replicating DNA. Much less work has been done on the removal of bound carcinogen; however, analysis of the removal of covalently bound mutagens and carcinogens is of great importance since this approach allows quantitation of the amount of damage actually removed during repair. To help assess the importance of repair synthesis in mutagenesis and carcinogenesis, we have begun to examine the removal of bound acetyl-aminofluorene (a potent chemical carcinogen and mutagen) and thymine dimers produced by ultraviolet radiation from a number of mammalian cell lines. Such studies should allow quantitative estimation of the extent of removal bound carcinogen and rates of removal of different adducts in different cell lines.